This invention relates generally to methods and apparatus for magnetic resonance imaging (MRI). In particular, it relates to radio frequency (RF) coil arrangements for generating the nuclei nutation field pulse and acquiring RF magnetic resonance signals when using an MRI apparatus that employs a vertical main magnetic field.
Magnetic Resonance Imaging (MRI) has become a widely accepted and commercially viable technique for obtaining digitized visual images representing the internal structure of objects (such as the human body) having substantial populations of atomic nuclei that are susceptible to nuclear magnetic resonance (NMR) phenomena. In MRI, nuclei in an object to be imaged are polarized by imposing a strong static main magnetic field, B0, on the nuclei. Selected nuclei are then excited (nutated) by imposing a radio frequency (RF) signal at a particular NMR frequency. By spatially encoding the selected nutated nuclei through a gradating of localized magnetic fields, and then suitably analyzing the resulting RF responses from the nuclei, a map or image of relative NMR responses as a function of the location of the nuclei can be determined. Following a Fourier analysis, data representing the NMR responses in space can be displayed on a CRT.
Commonly-known data acquisition techniques in MRI typically involve the utilization of localized RF receivers for acquiring NMR signals over a selected relatively small region of tissue. Conventional teachings suggest that efficient NMR signal reception requires the use an RF antenna coil configuration that has its greatest sensitivity in a direction that is substantially perpendicular to the main static magnetic field B0 of the MRI apparatus. An example of such localized NMR signal receivers are the conventional type of conductive loop antennae that are often used for imaging, for example, the spine of a supine patient (i.e. reclining face up) or the breast of a prone patient (i.e., reclining face down) laying within a horizontal main static magnetic field. In the case of human breast examinations, it is often desirable to obtain NMR signals from a tissue area that extends somewhat into the body or chest wall at the base of the breast. Through empirical trials it has been determined by the inventors that a loop or solenoid type coil disposed at the base of the breast most advantageously provides the desired signal components in such situations.
For example, a technique to image the breast of a patient is for the patient to lie prone and allow the breast to hang freely into a multiple loop or solenoid-type signal receiver coil. In this way, the breast axis is maintained vertical and the solenoidal coil form may conveniently be used to surround and secure the breast tissues during imaging. Since the primary magnetic field of a solenoid coil lies along its longitudinal axis, its greatest sensitivity to RF signals lies along this same direction. Accordingly, a solenoidal coil for breast imaging has the advantage that when imaging is performed in a MRI system having a horizontal main magnetic field, the direction of greatest sensitivity of the solenoidal coil is aligned verticallyxe2x80x94i.e., perpendicular to the main magnetic fieldxe2x80x94thereby allowing the coil to receive adequate MR signal levels from the chest wall of a prone patient.
When utilizing a MRI apparatus that employs a vertical main magnetic field, a xe2x80x9csaddlexe2x80x9d coil or a quadrature detection (QD) saddle coil pair is used for imaging breast tissues on a prone patient. This is because the saddle coil produces the majority of its magnetic field flux and has its greatest sensitivity in a direction perpendicular to its longitudinalxe2x80x94axis which in the above case of breast imaging on a prone patient would be aligned with the vertical main magnetic field. However, the magnetic field produced by a saddle coil does not extend sufficiently beyond the axial ends of the coil to significantly penetrate the chest wall. Consequently, it is usually not feasible to use a saddle coil or a QD saddle coil pair to obtain significant imaging data for tissue regions within the chest wall when imaging the breast in a vertical main field MRI apparatus.
Conventionally, a loop-type coil or a solenoidal coil is not oriented with its central axis oriented substantially parallel to the main static field of an MRI apparatus. It was conventionally anticipated that the NMR signal sensitivity of this type of coil would be at its lowest when its central axis is aligned parallel with the main field. Consequently, one would not have considered using a single loop coil or a solenoidal type coil for performing breast examinations on a prone subject in a MRI apparatus employing a vertical main magnetic field. Although the central B1 field of a loop or solenoidal coil may well be vertical, and thus give rise to no magnetization signal (NMR signals) from tissue regions within the coil (i.e., because the field inside the coil is parallel to the static main magnetic field B0), there are nevertheless substantial return flux field lines associated with a loop or solenoidal coil that lie beyond the axial end of the loop or solenoid which are directed at substantial anglesxe2x80x94including perpendicularxe2x80x94to the background B0 main field. The present invention developed from the inventors"" recognition that in such situations, these xe2x80x9coff axisxe2x80x9d effects associated with coil conductors can be put to productive use in an axial B0 field environment by allowing the production of a magnetization signal in tissue regions beyond the axial end of the coil.
In accordance with the present invention, productive use is made of the off axis effects associated with a loop or solenoidal coil(s) that is used in combination, for example, with one or more saddle coils in an RF coil array for NMR imaging. More specifically, the RF coils of such a multi-coil array are configured so that the longitudinal axes of the saddle coils are substantially parallel to the central axis of the loop (or solenoid) coil(s). The array is oriented within the imaging area of the MRI apparatus such that the central axis of the loop (or solenoid) coil is substantially parallel to the main B0 field. In this manner, the saddle coil(s) will elicit NMR signals from tissues within the coil array while the loop (or solenoid) coil elicits signals from tissue areas beyond the axial ends of the multi-coil array.
Although it is known to combine a saddle coil with a solenoidal coil in an RF coil array for MRI (see, for example, U.S. Pat. No. 5,293,519 to Yoshino et al. and U.S. Pat. No. 5,592,088 to Matsunaga et al.), such arrangements conventionally require that the loop or solenoidal coil(s) of the array be oriented with its central axis pointed perpendicular to the main magnetic field. Moreover, such known coil arrangements are physically designed to work only in this manner and are not suitable or adaptable for imaging the breast or spine in any other orientation or relation with respect to the main field.
Accordingly, one object of the present invention is to provide an efficient MW nutation/RF coil array arrangement specifically for use in vertical main field MRI systems and which is particularly suited for imaging regions of the breast extending into the chest in a prone patient or imaging the spine in a supine patient. A preferred embodiment contemplates an RF coil array arrangement comprising one or more RF coils that gather NMR signals in the conventional wayxe2x80x94i.e., by having their primary B field direction(s) oriented perpendicular to the MRI apparatus vertical magnetic fieldxe2x80x94combined with one or more single loop or solenoidal coils included in the array that are oriented having the central (longitudinal) axis aligned parallel to the vertical main magnetic field so as to also make use of the non-axial off axis components of the coil(s) to generate and receive NMR signal components from an extended region of tissue beyond the axial ends of the multi-coil array.
The present invention provides novel RF coil array arrangements for improving the magnetic resonance imaging of the breast or spine regions in prone (or supine) patients in a vertically oriented B0 field. The invention provides a method and apparatus for generating a nuclei nutation field pulse and for acquiring nuclear magnetic resonance signals when using an MRI apparatus that employs a vertical main magnetic field. In particular, the invention is directed toward a method and apparatus for utilizing the non-axial field components of an RF coil(s) to extend the imaging region during breast and spine image acquisitions in a vertical main field MRI system.
Accordingly, one embodiment of the present invention is an RF coil array for imaging a human breast in a vertical main field magnetic resonance imaging (MRI) apparatus. The coil array comprising a coaxial nested conductive coil pair consisting of a saddle coil connected in series with a loop-type or a solenoid-type coil. The loop or solenoidal coil is positioned at one axial end of the saddle coil. The central axis of the loop coil is oriented parallel to both a central longitudinal axis of the saddle coil and the vertical main magnetic field of the imaging apparatus.
Another embodiment of the invention is an RF coil array for magnetic resonance imaging (MRI) in a vertical main magnetic field imaging apparatus. The coil array comprises a coaxial nested conductive coil pair consisting of a saddle coil, a loop-type (or a solenoidal type) coil, and an analog combiner. The saddle coil is connected in an analog combiner arrangement with the solenoidal coil.
In a third embodiment of the present invention, an RF coil array includes a quadrature detection saddle coil pair used in combination with a loop-type or solenoidal coil to provide two signal channels. In a fourth embodiment of the present invention, an RF coil array includes a saddle coil and a loop-type (or solenoid-type) coil connected together in an analog combiner arrangement and used in combination with a second saddle coil to provide two signal channels. In a fifth embodiment of the present invention, an RF coil array employs a combination of a first saddle coil, a second saddle coil and a loop-type or solenoidal coil to provide three signal channels.